The use of security elements is well established in the field of security documents. One such type of security element is an optically variable device (OVD) which is a security feature or device that changes in appearance. OVDs provide an optically variable effect when the banknote is tilted and/or when the viewing angle of the observer relative to the OVD changes. The image of an OVD may also be changed by aligning a verification device over the security feature or device. An OVD may be provided by a printed area, e.g. an area printed with metallic inks or iridescent inks, by an embossed area, and by a combination of a printed and embossed feature. An OVD may also be provided by a diffractive device, such as a diffraction grating or a volume hologram and may include arrays of microlenses and lenticular lenses.
Lens-based security devices are a type of optically variable device (OVD). A description of cylindrical microlenses arranged in a lenticular array to make an OVD is provided in ‘Optical Document Security’, 3rd Edition by Rudolf L. Van Renesse ISBN 1-58053-258-6, pages 162 and 163, published 2005. The lenticular array is a series of cylindrical microlenses moulded in a polymer substrate. The microlens array can be placed on printed images and indicia such that when viewed from different angles, different images of the underlying indicia are seen. This technology can also be used with spherical or part-spherical microlenses and micro-images to produce magnified images by moire magnification, and to give an illusion of depth or movement as the underlying image is viewed from various angles.
Security elements, such as microlens arrays and other types of OVDs, such as diffraction gratings and volume holograms, are usually made separately from the remainder of the security document and subsequently applied to the document substrate to complete the security document. For example the OVDs are formed as a repeating pattern in a polymer substrate and wound into a roll. Then, in a completely separate process, the OVDs are applied to a designated area on the remainder of the security document.
Unfortunately, microlens structures and other types of security elements which have a structured surface, such as volume holograms, necessarily have a height or profile that gives the security document a significant localised increase in thickness. Typically the thickness can increase by about 30% or more in the area occupied by the security device. This localised thickness increase compounds when the security documents are stacked on top of each other. Similarly, if the security element is applied to a continuous web of the security documents and subsequently wound into a roll, the roll quickly develops a large eccentricity which generates downstream problems in the production process. The distortion in the roll causes some slack areas in the web and areas of excessive tension in the web where the web may burst or shows signs of “pressure blocking”. Pressure blocking occurs where two surfaces are brought into close contact with each other under pressure and, as a result of pressure surface entanglement or Van der Waal forces, the level of adhesion between the surfaces increases to such an extent that a significant force is required to part them. On occasion the force can exceed the cohesive strength of the two surfaces resulting in rupturing one or more of the surfaces.
If the security documents are a stack of separate sheets, the thickness variation in the stack creates complications when the sheets are fed or delivered through a sheet transport system. If the security document is a bank note, then a stack of the notes in a cash issuing and processing machine will form a wedge shape which limits the number of notes that can be processed at any one time. Portions of the stack can be regularly rotated 180° in order to produce a stack that is flatter. However, if the OVD is around the middle of the note, turning sections of the stack tends to be less effective at flattening the stack.
Wedge-shaped stacks limit the number of notes that can be processed at one time and increase processing costs. Rotating the notes within a stack introduces complexity and complications with note authentication. Furthermore, the production of the OVDs separately from the remainder of the document substantially increases the production costs relative to a security document compared with a type of security device that can be formed in a single ‘in line’ manufacturing process.